1. Field of the Invention
This invention pertains to a protective control unit for controlling output stage power semiconductor elements, the power semiconductor elements supplying current for an inductive load.
2. Description of the Related Art
As the control unit for controlling the power semiconductor elements at the output stages, which drives current into the inductive load, chopper circuits or H bridge circuits, which for example, control motions of the induction motors, etc. are known. In these circuits, protection circuits for protecting the output stage power semiconductor elements from a counter electromotive force (emf) generated in the inductive load is employed. For example, an H bridge circuit having the protection circuit is disclosed in Japanese Published Unexamined Patent Application 8-84060. The H bridge circuit has a highside-output transistor, which forms a highside-arm, and a lowside-output transistor, which forms a lowside-arm. The highside and lowside-output transistors are connected in series, and the inductive load such as motor, etc. is connected to the connecting point between highside and lowside-output transistors. A highside-driver is connected to base terminal of the highside-output transistor, and a lowside-driver is connected to base terminal of the lowside-output transistor. A voltage power supply Vcc is connected to collector terminal of the highside-output transistor, and emitter terminal of the lowside-output transistor is grounded. Between emitter-base terminals of the highside-output transistor, a highside-freewheeling diode is connected. Similarly, between the emitter-base terminals of the lowside-output transistor, a lowside-freewheeling diode is connected.
The breakdowns or destructions of the highside and lowside output transistors are prevented by passing currents due to the counter emf generated in the inductive load through these highside and lowside-freewheeling diodes. For example, the lowside-output transistor pass the current along the direction in which the current from the inductive load flows through the lowside-output transistor, when it is turned on by the lowside-driver. Afterwards, when the counter emf is generated in the inductive load, as the lowside-driver turns off the lowside-output transistor, the potential of the connecting point between the highside and lowside-output transistors rises. Then, the highside-freewheeling diode is forward biased, because the potential of the connecting point rises above the potential of the base terminal of the highside-output transistor, and the current flows in the highside-freewheeling diode. Thereafter, the highside-output transistor turns on in reverse-directional, and the return current due to the counter emf flows from the inductive load to the highside-output transistor.
However, the highside-output transistor becomes in a reverse recovery operation, while the highside-output transistor is turned on in the reverse-direction, when the lowside-output transistor turns on again. Then, the current flows from the inductive load along the direction to the lowside-output transistor again. Therefore, the charges stored in the highside-output transistor remain as they are. As this result, it is made to be a condition in which unrequired current flows in the forward direction of the highside-output transistor, i.e. from the collector to the emitter direction, though the highside-output transistor is supposed to be the off state. Namely, there is a problem that a large xe2x80x9cthrough-currentxe2x80x9d penetrating from the highside to lowside-output transistor flows in the conventional control unit for driving the power semiconductor elements having inductive load.
The purpose of the present invention is to provide a protective control unit for safely controlling the power semiconductor elements, serving as the output transistors, sufficiently pulling out the charges, which would be stored in the output transistors.
Another object of the present invention is to provide the protective control unit having lower power dissipation, and the miniaturization of the device size is easy.
Still another object of the present invention is to provide the protective control unit, in which the destruction or the breakdown of the output transistors due to the through-current penetrating from the highside to lowside output transistors is effectively prevented.
Still another object of the present invention is to provide the protective control unit, in which the turn-off time of the output transistors are shortened.
In view of above objects, a first feature of the present invention pertains to a protective control unit for controlling a highside-output transistor and a lowside-output transistor connected in series. The highside-output transistor has a first main electrode region connected to a power supply, a second main electrode region and a first control electrode. The lowside-output transistor has a third main electrode region connected to the second main electrode region, a fourth electrode region connected to ground and a second control electrode. And an inductive load is connected to a connecting point between the second and the third electrode regions. Namely, the protective control unit according to the first feature of the present invention inheres in a highside-drive circuit. The highside-drive circuit pulls out charges stored in the highside-output transistor, through the first control electrode, during the periods when the highside-output transistor is in the end of reverse conducting state and reverse recovery state.
A second feature of the present invention pertains to a protective control unit for controlling the highside-output transistor and the lowside-output transistor connected in series. That is, the second feature of the present invention lies in a charge removing means for pulling out charges stored in the highside-output transistor, through the first control electrode, during the periods when the highside-output transistor is in the end of reverse conducting state and reverse recovery state.